Single pole insulated voltage transformers used in electricity distribution networks may be equipped with three windings. In addition to a primary winding, transformers have a secondary winding, for example, a measuring winding, which is used for either measuring or protection purposes, and a tertiary winding, for example, an earth-fault winding, which is utilized for earth-fault protection purposes. The terminals of the primary, measuring and earth-fault windings are generally denoted as: A-N, a-n and da-dn, respectively. FIG. 1 shows a terminal diagram of a single pole insulated voltage transformer with three windings. It is known for all three windings to be wound around the same magnetic (iron) core. FIG. 2 illustrates a principal construction of an exemplary single pole insulated voltage transformer with three windings, including a primary winding 20, an earth-fault winding 30 and a measuring winding 40, which are wound around a core 50 and enclosed in a housing 60. The fact that all the windings 20, 30, 40 are wound around the same magnetic core 50 makes the windings interlinked through magnetic fluxes. Consequently, when a primary voltage is applied to the primary winding 20, the secondary windings 30, 40 produce secondary voltages that depend on the primary voltage and a turns-ratio between the secondary winding in question and the primary winding.
In a three-phase network, the earth-fault windings of three single pole insulated voltage transformers may be connected in an “open-delta” connection. This is due to the fact that during an earth-fault in the primary network, the voltage between the open-delta terminals is related to a residual voltage of the network (voltage between earth and neutral point of the three-phase system). This voltage is utilized in earth-fault protection relays. In addition, a resistor may be connected between open-delta terminals in order to provide necessary damping power in case of ferro-resonance. FIG. 3 shows a known configuration of three single pole insulated voltage transformers 11, 12, 13 each with primary windings 21, 22, 23, measuring windings 41, 42, 43 and earth-fault windings 31, 32, 33 connected to phases PA, PB, PC of a three-phase network. Earth-fault windings 31, 32, 33 of the three single pole insulated voltage transformers 11, 12, 13 are connected in open-delta connection. A resistor Rd is connected between open-delta terminals in order to provide necessary damping power in case of ferro-resonance. The primary voltages of the three phases PA, PB, PC of the three-phase network may then be obtained on the basis of the secondary voltages measured from the measuring windings 41, 42, 43 and the turns-ratio between the primary winding and the measuring winding. However, due to various non-idealities of the transformers, the primary voltages obtained merely on the basis of measured secondary voltages and the turns-ratio between the primary winding and the secondary winding may not accurately correspond to true primary voltages.
WO 2009/091803 discloses a transformer with two secondary windings. In WO 2009/091803, the transformer includes a compensation circuit which includes a current transformer and an impedance in order to improve an accuracy of a metering voltage by compensating for a voltage drop across primary winding incurred by a load current of a power load connected to a secondary winding. A drawback of the disclosed solution is that it requires an additional current transformer and an impedance to be added to the transformer structure.